FIG. 4 is a block diagram showing one example of a previously-known fully-closed position controller (hereinafter referred to merely as “position controller”) that controls a position of a movably-formed target system 112 which comprises a movable member such as a table and is joined by coupling or the like to a servo motor (not shown) used as a drive motor.
The configuration and operation of the position controller are described below. The position controller receives an input of a position command value X from a higher-level device (not shown). A subtractor 101 calculates a positional deviation X−xL by subtracting a position detected value xL from the position command value X. The position detected value xL is a position detection signal which is obtained by directly detecting the position of a control target of the target system 112 by a linear scale or the like (not shown).
Further, a time derivative of the position command value X is obtained by differentiators 104, 109 which respectively output the obtained values as a speed command value V and an acceleration command value A. The acceleration command value A is amplified by an acceleration torque conversion constant Ca by an amplifier 110 to be used as an acceleration torque command value τca that causes the target system 112 to be moved at the rate of acceleration. The speed command value A and the acceleration torque command value τca are respectively added to the speed command and the torque command. Such a series of processes form a well-known feed forward block for reducing a constant positional deviation X−xL down to zero.
An amplifier 102 proportionally amplifies the positional deviation X−xL at an amplification factor of a position loop gain Gp. An output from the amplifier 102 becomes the final speed command value Vc after the speed command value V is added to the output by an adder 103. A subtractor 105 calculates a speed deviation Vc−vm by subtracting a motor speed vm from the speed command value Vc. The motor speed vm is a time derivative value, obtained by a differentiator 107, of a rotational angular position xm of a position detector (not shown) connected to a servo motor, or an output of a speed detector (not shown) connected to the servo motor. An amplifier 106 amplifies the speed deviation Vc−vm at an amplification factor of a speed loop gain Gv.
The output of the amplifier 106 becomes a torque command value τc after addition of an acceleration torque command value τca by an adder 108. The torque command value τc becomes a generated torque τ for the target system 112 after being power amplified by a power amplifying unit 111. The power amplifying unit 111, which consists of a power amplifier and a servo motor, amplifies the torque command value τc to output the generated torque τ. The amplification ratio is represented by a torque conversion constant Ct. The generated torque τ is supplied to the target system 112 and used to drive the target system 112. It should be noted that, a reference symbol “S” of the differentiators in FIG. 4 represents a Laplace transform operator indicating a differential operation.
In previously-known position controllers, a positional follow-up deviation due to a frictional force is minimized by providing a friction compensation calculation unit 113 to respectively add compensation values Vsfc, τsfc calculated by the friction compensation calculation unit 113 to a speed command and a torque command.
However, when sliding characteristics of a target system are changed due to deterioration over time, a temperature change, a lubrication state change of a sliding surface, or the like, desirable friction compensation becomes impossible, causing a positional follow-up error and a decrease in processing accuracy. In order to avoid this, control parameters of the friction compensation calculation unit 113 should be reset (re-adjusted). Because resetting is time-consuming and is required every time the sliding characteristics change, resetting is troublesome.
Further, although changing control parameters of the friction compensation calculation unit 113 in accordance with a change in the sliding characteristics may be considered, because the sliding torque changes according to a traveling speed of the target system, it is necessary to compare sliding torques under the same speed in order to quantitatively handle the sliding torques. Therefore, it becomes necessary to obtain sliding toques by providing a dedicated operation mode, which would result in an increase of non-cutting time.
The present invention is provided to overcome these problems. An object of the present invention is to provide a position controller which can maintain desired friction compensation without a need to provide a dedicated operation mode even when sliding characteristics of a target system change due to deterioration over time, a temperature change, a lubrication state change of a sliding surface, or the like.